WO1990010867A1

WO1990010867A1 - Screening system

Info

Publication number: WO1990010867A1
Application number: PCT/GB1990/000389
Authority: WO
Inventors: Anthony Charles Minson
Original assignee: Immunology Limited
Priority date: 1989-03-15
Filing date: 1990-03-15
Publication date: 1990-09-20
Also published as: CA2012088A1; CN1045603A; EP0463046A1; JPH04505964A; AU5286090A; US5045447A

Abstract

A screening system for use in a method for the production of antibodies to an immunogen, wherein hybridoma culture supernatants are screened using cells infected with a virus recombinant encoding one or more epitopes of the immunogen, which infected cells are first subjected to one or more procedures conventional for the preparation of clinical samples to be arranged for the presence of said epitopes. The system disclosed is particularly advantageous as 1) cells infected with the virus recombinant can provide a good supply of an antigen; 2) the antigen is expressed in a mamalian cell and it is therefore synthesized and processed in a form essentially identical to the antigen found in a natural infection; and 3) because the infected cells are subjected to one or more pre-treatments as applied to test sample tissues before their use in the hybridoma screening procedure, this ensures that the antibodies selected will recognize epitopes resistant to these pre-treatments and which may therefore be expected to be present on the test samples as prepared for analysis.

Description

SCREENING SYSTEM

Field of the invention

The present invention relates to a screening system. In particular it relates to a screening system for a method for producing antibodies which are screened for their ability to bind to a specified antigen. The invention also relates to methods and materials for carrying out the screening system; to antibodies produced using the screening system; to diagnostic procedures and kits utilising such antibodies; and to kits utilising said materials for carrying out the screening system as assay controls.

The present invention is exemplified herein, by way of example only, and not by way of limitation, to the production of monoclonal antibodies to human papilloma virus (HPV) by use of such a screening system, although it will be obvious to those skilled in the art that the techniques described may be usefully employed in many antibody production systems.

Background to the invention

For successful use of an antibody in a diagnostic test the antibody must recognize an epitope which is common to the immunogen and the test sample as prepared for analysis (i.e. after any pre-treatment of tissues such as cryopreservation, sectioning and fixing). Therefore, the system chosen for screening large numbers of e.g. hybridoma culture supernatants must be such that it aids selection of diagnostically useful antibodies.

Up until now the production of antibodies to a given antigen e.g. HPV (for research purposes) has been achieved using as immunegens either proteins expressed in bacterial expression systems, proteins in the form of synthetic oligopeptides or purified proteins, and screening (e.g. the hybridoma supernatants) for antibodies produced against the immunogens by reusing the immunogens or various combinations thereof (Banks L. , et al 1987, J. Gen. Virol. Vol 68., 3081-3089; Doorbar J. and Gallimore P., et al 1987, J. Virology 6^, p.2793-2799).

However, these known protocols for the production of monoclonal antibodies are generally unsuitable for the production of monoclonal antibodies

_* which are to be used in immunocytochemical diagnostic tests. This is because antibodies produced by these protocols will not necessarily react with the naturally occurring protein in infected human cells. And more particularly, the epitopes recognized by these antibodies will not necessarily be those epitopes which are resistant to the standard procedures involved in the sampling, fixing and storing of clinical specimens. The present inventors have realised that in order to select an antibody which is diagnostically useful, it is desirable that the screen material closely mimics a clinical sample. In particular the screen material must present the antigen in the same form as presented by the clinical sample during diagnostic assay.

Before a clinical sample such as a cell smear or tissue section is used in an antibody based diagnostic test, samples are generally subjected to one or more pre- treatment steps. These pre-treatment steps may comprise cryopreservation, sectioning, and/or fixing (e.g. by methanol or formaldehyde/saline) . Because of these procedures, an antigen expressed by clinical sample subjected to such pre-treatment steps may be somewhat modified when compared to the antigen expressed in vivo.

The inventors have therefore realised that in any screening system it is desirable thaτ the screen material mimics the clinical sample in two main ways. Firstly the screen material must be able to produce, process and present the material as naturally as possible. Secondly, the screen material must be such that it can be subjected to the same pre-treatment steps as are routinely applied to_. clinical material before diagnostic analysis, before the screen material is used in a screening system.

As mentioned above, the invention is exemplified with reference to human papillo a virus.

Human papilloma viruses are small (approximately 8 kb) DNA viruses and more than 40 types have now been reported as inducing epithelial or fibroepithelial proliferations of the skin or mucosa (zur Hausen, H., 1977, Current Topics in Microbiol. and Immunol. 7*3, 1-30). In particular the DNA of several HPV types is found in a variety of genital lesions, ranging from benign warts (both common and genital) which often contain HPV 6b or HPV 11 DNA (Gissmann L. et al, 1983, Proc. Nat. Acad. Sci. (USA) 8 , 560-563), to invasive squamous cell carcinomas of the cervix, which frequently harbour HPV -16, -18, -33 or -35 genomes (Durst M. , et al 1983, Proc. Nat. Acad. Sci. USA 80, 3812-3815; Beaudenon S., et al 1986, Nature, Vol. 321, 246-249).

From the cytological examination of cervical smears, it is possible to designate up to 10% of the women from whom the smear samples were taken, as having abnormal cytology. Such women are then referred for further physical examinations, which frequently involve invasive diagnostic procedures. A proportion of these women are infected with non-serious HPV while others are infected with more ominous HPV types. Thus in a screening system it would be useful to have a diagnostic test which both identifies HPV infection as being present, and distinguishes non-serious from serious HPV infection. In order to minimise equipment and training costs it would be advantageous for any new diagnostic test to utilise existing technology and slpt easily into the present screening procedure. Furthermore, the provision of objective answers (rather than subjective assessments) by any such tests would allow automation, and this in turn would help to reduce both costs and waiting times, the latter being frequently stressful for the patients concerned. To date no antibody-based clinical diagnostic tests are available for the detection of HPV infection. This is largely due to two problems. Firstly, it has not been possible to date to establish a permissive tissue culture system for the in vitro propagation of papilloma viruses. Secondly, clinical material is difficult to obtain, the viral proteins are usually present in only very small quantities in the clinical lesions, and the HPV causing the lesion is generally of unknown type. These two factors mean that a reliable and sufficient source of typed (i.e. known) HPV is not available for use as both an immunogen and screening agent in the large- scale production of monoclonal antibodies. Summary of the invention

The invention provides a screening system for use in a method for producing antibodies which have an ability to bind to a specified antigen, wherein the antibodies are screened with cells infected with a recombinant virus vector expressing an antigen specific for the desired antibody, which cells have been pre- treated in a similar way to the preparation of clinical samples. Preferably, the antibodies are obtained from an animal which has been immunised with a protein expressed from recombinant DNA in suitable host organisms or with a synthetic oligopeptide, said protein or oligopeptide presenting an epitope homologous to an epitope encoded by the viral vector.

The term "homologous" in connection with epitopes is used to indicate that the epitopes are represented by substantially the same amino-acid sequence, while recognising that because of their different manner of production they may not be identical in terms of their conformation. The screening system of the present invention enables antibodies to be identified which recognise common structural features of the homologous epitopes, and are therefore less likely to be affected by any conformational changes in the epitope presented by a prepared clinical sample. Antibodies obtained by use of the screening system can be further screened against prepared clinical samples of known antigenicity (e.g. infected with HPV of known type), to ensure that the antibody has the required ability to recognise the epitope in that situation. The invention also provides screening material for use in a screening system as herein provided, which comprises cells infected with a recombinant virus vector expressing an antigen specific for the desired antibody, which cells have been pre- treated in a similar way to the preparation of clinical samples.

The invention also provides antibodies produced by use of the described screening systems. Also provided are diagnostic kits which incorporate either or both of: a) antibodies produced by use of the described screening systems and b) screening material which comprises cells infected with a recombinant virus vector together with one ^• or more reagents necessary for performing an assay on a clinical sample for the presence of a protein which presents an epitope to which the antibody is specific and wherein the recombinant virus vector expresses an epitope homologous to that presented by the protein. The cells of the screening maτerial may be pre-treated in a similar way to the preparation of clinical samples. In such kits, the screening material may be incorporated for use as an assay control.

The invention also includes a diagnostic procedure, in which an antibody produced by use of the described screening system is used to assay a clinical sample for the presence of a protein which presents an epitope to which the antibody is specific.

The method can be used for the production of antibodies which are able to distinguish between different strains of an immunogen. This may be achieved by screening the antibodies using cells infected with virus recombinants encoding one or more epitopes specific to a particular strain of immunogen. A suitable virus vector is vaccinia. The method disclosed is particularly advantageous as: 1) cells infected with the ^: virus recombinant can provide a good supply of an antigen; 2) the antigen is expressed in a mammalian cell and it is therefore synthesized and processed in a form essentially identical to the antigen found in a natural infection; and 3) the infected cells are subjected to the same pre- treatments as applied to test sample tissues before their use in the hybridoma screening procedure. This ensures that the antibodies selected will recognize epitopes resistant to these pre-treatments, and which may therefore be expected to be present on the test samples as prepared for analysis. Typically, the pre- treatments may comprise pelleting the infected cells by centrifugation, fixing, and preparing as a section. The fixing procedure would typically be by methanol where the clinical sample is to be a cervical smear, or by formaldehyde/saline where the clinical sample is to be biopsy material.

The screening system and the virally-infected cell screening material as provided herein, can be used for the rescreening of antibodies, however they may have been initially prepared and selected.

Apart from papilloma virus, which provides problems as already indicated, the invention is applicable more generally. Especially in relation to antibodies for an antigen that is difficult to produce routinely in a native form.

A particular advantage of using virally infected cells as a screen or positive control, is that they can be prepared to present the antigen at known levels, for example equivalent to 1 cell in 10,000. Thus, one can judge, the suitability of an antibody for a particular assay, or the strength of a positive signal in the assay using the positive control.

Brief descriσtion of the drawinσs

Figure 1 shows the construction of an expression plasmid pHX2 for a fused β-galactosidase HPV16-L1 open reading frame; -

Figure 2 shows the construction of a full length HPV16-L1 open reading frame in pUC18 (see also Browne et al 1988 J.Gen.Vivo1.69, 1263-1273); Figure 3 shows the construction of a vaccinia expression plasmid pRK19, an insertion vector for expressing coding sequences under the control of the vaccinia virus 4b promoter (see also Richard Kent PhD Thesis, University of Cambridge);

Figure 4 shows the construction of an expression plasmid pRKLl from pRK19, in which the HPV-16 LI gene is controlled by the vaccinia virus 4b promoter (see also Browne et al 1988 J.Gen.Vivol. 69, 1263-1273); Figure 5 shows the construction of an expression plasmid pExE7 for-a fused β-galactosidase HPV- 16 E7 open reading frame;

Figure 6 shows the construction of an expression plasmid pRKE7 in which the HPV16 E7 gene is under the control of the vaccinia virus 4b promoter; and

Figure 7 shows schematically the use of a screening system according to the present invention. Detailed description of preferred embodiments of the invention In order that the present invention may be more readily understood, an embodiment will now be described, by way of example only and not by way of limitation. 1. Production of monoclonal antibodies to HPV-16 LI (a) Preparation of HPV-16 Ll/β-galactosidase fusion protein (the immunogen)

A genomic clone of HPV-16 DNA was obtained (for details of the complete nucleotide sequence see Seedorf et al 1985, Virology 145, 181, incorporated herein by reference). A portion (amino acid 211 tcr C-terminus) of the HPV-16 LI open reading frame was cloned as a Bam Hl/Sphl fragment (bases 6153-7464) from a genomic clone of HPV-16 DNA and ligated into BamHl and Sail sites of the vector pEx-1 (Stanley & Luzio 1984; EMBO J. 3, 1429, incorporated herein by reference), to yield a fused open reading frame of β-galactosidase and the C-terminal portion of HPV-16 LI (Fig. 1). The resulting plasmid (pHX2) was transfected into E.coli POP 2136 and heat induction resulted in the production of a β-gal fusion protein that was purified by gel electrophoresis.

(b) Production of monoclonal antibodies to HPV-16 LI.

Mice were immunised with three intraperitoneal injections of 100 μg of rhe β-galactosidase-Ll fusion protein (prepared in (la) above), in incomplete Freunds adjuvant at monthly intervals. Four days after the final immunisation mouse spleen ceils were fused with myeloma NSO cells and the fusion products were distributed among 48 x 1.5 cm diameter tissue culture wells. (c) Prepration of Screening Material

(i) Preparation of a recombinant vaccinia virus expressing the full length HPV-16 LI protein

The HPV-16 LI open reading frame was introduced into the vector pUC18 in a Kpnl - Sphl fragment (bases 5377-7464) derived from an HPV-16 genomic clone (Fig 2). The open reading frame was then excised from this vector in a 2074 bp fragment resulting from a partial EcoRl digestion and this fragment was introduced into the EcoRl site of pRK19, a vector containing a vaccinia late promoter (the promoter of the 4b late gene) flanked by vaccinia thymidine kinase coding sequences (R. Kent, Ph.D. thesis, Cambridge 1988 incorporated herein by reference but summarised below).

The resulting plasmid, pRKLl (Fig 4), contains the entire HPV-16 LI gene under the control of the vaccinia late promoter. pRKLl was transfected into CV-1 cells infected with wild-type vaccinia virus and recombinant viruse, lacking an intact thymidine kinase gene, were selected from the progeny by growth in 5- bromodeoxyuridine. Recombinant viruses were then identified by hybridisation with a HPV-16 DNA probe and further characterised by restriction enzyme digestion. A recombinant virus containing the HPV-16 LI gene inserted in the correct orientation was identified, and named vLlRK.

The procedures outlined in (1) and (2) above are described more fully in Browne et al 1988 J. Gen. Virol. 69_, 1263-1273, incorporated herein by reference.

The steps in the construction of a plasmid containing the 4b gene promoter are as follows (See Fig 3):

(a) The 4b gene coding sequence and 228 nucleotides of 5' sequence (containing the promoter and transcription start site) were isolated on a 2.3 kb Xbal fragment derived from the vaccinia virus (WR strain) HIndlll A restriction fragment. The 2.3 kb Xbal fragment was cloned into the Xbal site of M13 mpl9, and the majority of the 4b coding sequences were deleted by Xhol cleavage, Bal 31 digestion and religation. The resulting insert in M13 mpl9 contained 228 nucleotides of 'upstream' sequence and 31 nucleotides of 4b coding sequence.

(b) The remaining 4b coding sequences were deleted by site-directed mutagenesis so as to bring the cloning sites in M13 mpl9 immediately 3' of the 4b promoter and transcription start site. The sequence in this region now reads

Bam Sma Kpn etc. CGAATATAAATAAGGATCCCGGGTACC Vaccinia 4b promoter M13 mpl9 cloning sequences sites

Finally, pRK19 was constructed by removing these sequences from the vector (by EcoRl and Accl digestion) and inserting them into the body of the vaccinia thymidine kinase gene in a vector analogous to pGS20 (see Mackett et al., 1984, Journal of Virology 49, 857, incorporated herein by reference). pRK19 thus has the properties given in Figure 3. This promoter was used because it was conveniently available, but in principle any of the vaccinia late promoters could have been used. The features of 'late' vaccinia promoters have been described by a number of groups e.g., Rosel et al. 1986, Journal of Virology 0, 436 and Betholet et al., 1986, EMBO Journal 5., 1951, both incorporated herein by reference. There are several examples of the use of these promoter sequences to drive __ the expression of foreign proteins in vaccinia virus recombinants e.g. Bertholet et al., 1984, PNAS 82, 2096 and Miner et al., 1988, Journal of Virology 6_2^ 297, both incorporated herein by reference and the vectors described by these authors would also be suitable for expressing HPV coding sequences. The 4b gene promoter has the features typical of other 'late' promoters. The 4b gene and its promoter region have been sequenced by Rosel and Moss 1985, Journal of Virology Jij5, 830, incorporated herein by reference. ii) Preparation of vaccinia infected cells as pellets for formalin fixation, paraffin wax embedding and tissue sect —ion for evaluati—on... of... monoclonal antibodv— reactivity

Preformed monolayers of target cells (10^ or more ) were infected with recombinant vaccinia virus or wild type vaccinia virus (negative control) and infection was allowed to proceed for 8-15h. Baby hamster kidney cells (BHK-21) and Vero cells have also been used, and in principle any susceptible cell line would suffice. The multiplicity of infection (MOI) is varied depending on the proportion of antigen positive cells required. An MOI of 5 results in 100% +ve cells. An MOI of 0.1 results in 10% +ve, and the MOI can be infinitely varied to give a proportion of +ve cells defined by the Poisson distribution. For use as screening material, the MOI should be high e.g. 70-100%.

The^' cells were then scraped off with a teflon cell scraper and collected in a small volume of PBS (Dulbeccos A). The suspension of cells was then spun at approximately 200 rpm in a bench centrifuge for 10 minutes. After removal of the supernatant fluid, the pellet was resuspended in 1.0 ml of PBS and transfered to an Eppendorf tube. The cell pellet resuspension was then spun in a microfuge at maximum speed for 5 minutes. The supernatant fluid was then removed and replaced with 1.0ml of 5% buffered formol-saline and left at room temperature for 5-60 minutes. Using a fine syringe needle, the solid pellet was carefully displaced so that it floated in the fluid. The conventional protocols for dehydration and paraffin wax infiltration as are well known in the art were then proceeded with using the small solid pellet and treating it as a small biopsy. Sections for immunodiagnosis were cut using standard protocols.

(iii) Preparation of vaccinia infected cells as cell smears for evaluation of monoclonal antibody reactivity.

Preformed monolayers of target cells (10^ or more) were infected with recombinant vaccinia virus or wild type vaccinia virus (negative control) and infection was allowed to proceed for 8-15 h. Baby hamster kidney cells (BHK-21) and Vero cells have also been used and in principle any susceptible cell line would suffice. The multiplicity of infection (MOI) is varied depending on the proportion of antigen positive cells required. An MOI of 5 results in 100% +ve cells. An MOI of 0.1 results in 10% +ve and the MOI can be infinitely varied to give a proportion of + ve cells defined by the Poisson distribution.

The cells were scraped from the growth flask with a teflon scraper and collected in a small volume of PBS (Dulbeccos A) and spun at 2000 rpm in a bench centrifuge for 5-10 minutes. The supernatant fluid was removed and the wash repeated. Finally enough PBS was added to bring the cell suspension to a concentration of 10 cells/ml and the cells were resuspended. Using a pasteur pipette, a drop of the cell suspension (lOOμl was placed on a glass slide). The slide had been pre-treated with 3-aminopropyltriethoxysilane (APES) to ensure cell adherence. Then using either a conventional wooden or plastic spatula (used to take cervical" smears) or a bacteriological swab or loop the cell droplet was smeared evenly across the slide. The smear was immediately spray-fixed with 90% ethyl alcohol (v/v) or placed in 90% ethyl alcohol for 5 mins. The smear was then left to air dry. Just prior to iπununostaining these smears were placed in 5% buffered formol saline for 5 minutes, then rinsed twice in PBS before proceeding with immunostaining protocols as are well known in the art. In the present example, culture supernatants were screened for HPV-16 Ll-specific antibody by immunofluorescence assay using as a target BHK-21 (fibroblasts infected with vLlRK. (d) Screening

Sections (or cell smears) produced as described, and comprising infected cells with a high MOI were then used as targets in the antibody screening. Hybridoma culture supernatants were incubated on the sections for 30 mins at room temperature and, after washing in PBS, bound immunoglobulin was detected by incubation with fluorescein-conjugated rabbit anti-mouse IgG. A positive culture was identified on the basis of strong nuclear fluorescence and the cells in this culture were subjected to two rounds of limiting dilution cloning. The resulting hybridoma was named CAMVIR-1. The secreted antibody was found to immunoprecipitate a protein of apparent Mr of about 55,000 from lysates of cells infected with vLlRK. This protein was not precipitated from lysates of cells infected with wild- type vaccinia virus. This apparent Mr was consistent with the predicted Mr of the HPV-16 LI protein of about 53,000. The antibody was also found to give strong nuclear staining (using immunofluorescence and/or immunoperoxidase staining) of cells infected with vLlRK and fixed with formaldehyde or methanol.

The fact that the monoclonal antibodies produced by hybridoma CAMVlR-1 gave strong nuclear staining, indicates .that the expression of epitopes expressed by cells infected with VLlRK were not being destroyed by procedures routinely applied to clinical samples.

The use of a cellular material infected with recombinant vaccinia virus as a source of target antigen which has been treated in a manner comparable to pre- assay treatments as applied to clinical samples, thus allowed the selection of a diagnostically useful antibody with the required specificity. The validity of these tests was subsequently confirmed by immunoperoxidase staining of clinical specimens. Antibody CAMVIR-1 detected HPV antigen in the cell nuclei of biopsy sections and cervical smear preparations known to be positive for HPV-16 (assessed by nucleic acid hybridisation) but not in specimens known to be positive for HPV-6 or HPV-11.

2. Production of monoclonal antibodies to HPV-16 E7 A similar approach was used to prepare two further hybridomas, CAMVIR-2, -3 and -4, which produce monoclonal antibodies against the E7 protein of HPV-16. Cloning and expression details for the HPV-16 E7 gene are as follows. (a) Preparation of a β-galactosidase-E7/ -galactosidase fusion protein (the immunogen).

A genomic clone of HPV-16 (Seedorf et al, supra) was digested with Pstl and the resulting 1776 bp fragment (bases 7003-875), which includes the E7 open reading frame, was cloned into the Pstl site of pUC13. The resulting plasmid pJS2 (see Fig. 5) was digested with Pstl and Nsil and a 313 bp fragment (nucleotides 562-875) was cloned into the Pstl site of pEx-1 (Stanley and Luzio, supra) to generate pEXE7 containing a fused β- galactosidase-E7 coding sequence. The expressed product was induced and purified as described for the HPV-16 LI fusion protein (and Browne et al, supra), (b) Production of the HPV-16 E7 monoclonal antibodies

The β-galactosidase-E7 fusion protein (prepared in 2 (a) above) was used to immunise mice as described for the HPV-16 LI fusion protein in section 1(b). (c) Preparation of screening material

(i) Preparation of a recombinant vaccinia virus expressing HPV-16 E7 protein

Plasmid pJS2 (Fig. 5) was linearised with Hindlll and partially digested with PvuII. A 335 bp fragment originating from the pUC13 Hindlll site and terminating at the HPV-16 PvuII site (nucleotide 553) was purified, end repaired and cloned into the Smal site of pRK19 (see Fig. 6) such that the E7 gene is expressed under the control of the vaccinia 4b late promoter. Transfection of CV-1 cells and isolation of a recombinant, thymidine kinase negative virus, was as described for the production of an Ll-recombinant (Mackett et al, supra). The recombinant vaccinia virus expressing HPV-

16E7 protein was then used as described in section 1(c) (ii) and/or 1(c) (iii) and 1(d) for HPV-16 LI to produce a diagnostically useful antibody against HPV-16E7. Except the cells used in screening were fixed briefly' (i.e. an hour or less) in formalin.

Figure 7 shows ^'schematically the use of a screening system for the selection of diagnostically useful antibodies. A recombinant virus vector containing a gene sequence encoding the antigen of choice is used to infect a sample of eukaryotic cells. After a suitable incubation, these infected cells then produce, express and present the antigen. The cells are then collected and formed into a cell pellet. This cell pellet can then be treated as a small clinical biopsy (or used as a source of cells to prepare a cell smear). Thus, sections are taken and processes as are routinely applied to clinical samples before diagnostic analysis, are also applied to the e.g. sections formed from the infected cells. These processes or pre-treatment steps may alter the nature of the presented antigen. As these pre- treatment steps are applied to clinical material the antigen as naturally expressed may be similarly altered.

Hybridoma supernatants can then be tested on the eg cell pellet sections to search for diagnostically useful antibodies which bind to the antigen in a form that might be expected to be present in a clinical specimen. Antibody-antigen-binding is detected by techniques well known in the art e.g. use of a fluorescein, enzyme or radio labelled second antibody.

A pellet section or cell smear comprising cells infected with a recombinant virus vector containing a gene sequence encoding an antigen of interest, can be usefully incorporated as a control in a diagnostic assay kit. For this use, the. cells infected with the recombinant virus vector should have a low MOI e.g. 3% or less. For example, the control may be provided in the form of a microscope slide or the like, bearing a section of the cell pellet comprising the infected cells. The cells will be known to be presenting the antigen of interest. Therefore, the control will be treated as a test sample and because the control is known to be presenting the antigen of interest it should •produce a positive test result. If such a control does not produce a positive test result, this would indicate the assay is defective and that any negative results obtained for the samples may not be necessarily true negatives. The methods exemplified above are generally applicable to other antigens and to the use of other viral vectors.

Claims

1. A screening system for use in a methβd of producing an antibody which has an ability to bind to a specified antigen, wherein the antibodies are screened with cells infected with a recombinant virus vector expressing an antigen specific for the desired antibody, and which infected cells are first subjected to one or more procedures conventional for the preparation of clinical samples to be assayed for the presence of said antigen.

2. The screening system of claim 1, wherein the virus vector is vaccinia.

3. The screening system of claim 1 wherein the antigen • is a polypeptide comprising an epitope of human papilloma virus (HPV).

4. The screening system of claim 3, wherein the epitope is comprised in the HPV-16 LI protein or HPV-16 E7 protein.

5 . The screening system of claim 1 wherein the antibodies are obtained from an animal which has been immunised with a protein expressed from recombinant DNA in a suitable host organism or with a synthetic oligopeptide, said protein or oligopeptide presenting an epitope homologous to an epitope encoded by the viral vector.

6. The screening system of claim 1 wherein the procedures conventional for the preparation of clinical samples comprise one or more of pelleting, sectioning, dehydrating and fixing.

7. The screening system of claim 1 wherein antibodies selected by said screening system are then subjected to further screening against one or more prepared clinical samples known to contain the antigen.

8. The screening system of claim 1 wherein antibodies are first preselected by subjecting them to a first screening procedure which may be different to the screening system of claim 1.

9. A diagnostic procedure in which an antibody selected by a screening system according to any one of the preceding claims is used to assay a clinical sample for the presence of a protein which presents an antigen to which the antibody is specific.

10. A screening material which comprises cells infected with a recombinant virus vector expressing an antigen specific for the desired antibody, which infected cells have been subjected to one or more procedures conventional for the preparation of clinical samples to be assayed for the presence of said antigen.

11. An antibody, derivative thereof or functional equivalent, which is selected by a screening system according to any one of claims 1 to 8.

12. A diagnostic kit which comprises either or both of: a) an antibody, derivative thereof or funtional equivalent according to claim 11;

b) screening material which comprises cells infected with a recombinant virus vector expressing an antigen specific for the desired antibody; together with one or more reagents necessary for performing an assay on a clinical sample for the presence of antigen to which the antibody is specific.

13. A diagnostic kit according to claim 12 wherein the screening material can be utilised as a positive control for the assay.

14. A diagnostic kit according to claim 12 wherein the cells comprising said screening material have first been subjected to one or more procedures conventional for the^" preparation of clinical samples to be assayed for the presence of said antigen.

15. A diagnostic kit according to claim 14, wherein the procedures comprise one or more of pelleting, sectioning, dehydrating and fixing.

16. A diagnostic kit which comprises:

(i) an antibody specific for a given antigen;

(ii) reagents for using that antibody to detect an analyte in a clinical sample;

(iii) a positive control specimen comprising cells infected with a recombinant viral vector expressing said antigen; and

(iv) reagents for detecting the binding of said antibody

(i) with the antigent in the speciment (iii).

17. A diagnostic kit according to claim 16 wherein the reagents (iv) are to be included in reagents (ii) .

18. A diagnostic kit according to claim 16 wherein the specimen (iii) is treated in a manner analogous to that appropriate for the clinical sample.